I was desperately grasping at straws, and to no avail. The OE pump did not change a thing. I am completely stupefied by this car.

All I can hope for is that one of the ‘Super Techs’ here can pipe in on this exasperating situation. There is no substitute for experience. Someone that has witnessed the idiosyncrasies of dozens or even hundreds of these cars may have an idea. Perhaps one of their customers has had a similar problem; perhaps it is an early sign of greater problems yet to come, such as a head gasket in the early stages of failure? I don’t know.

Additional information: This does not seem to be a problem with cooling capacity per se. For example, when the gauge was reading about 98, I checked the temperature of the *hot* side of the thermostat housing, the infrared thermometer read 99 degrees. The (cooled) water return pipe going from the bottom of the radiator to the thermostat housing reads between 60-65 degrees; so the radiator is cooling the coolant. This would seem to indicate that the thermostat is not opening properly, but I’ve now had THREE different thermostats in the car. What are the odds of 3 bad thermostats in a row? Rather astronomical. If only I had such ‘luck’ with those odds consistently, I’d play the lottery!

To answer the last few replies:

Donkey: The gauge is definitely reading accurately. Heater core is good, but that is not a factor in this type situation. Diesel grade oil, absolutely; but again not a factor here. As far as $$$ of guesswork – well, I hate guessing. That is why I try to understand the principles as well as the specifics. The only reason that everything was replaced was due to my practice of replacing the hoses & water pump on any car that I purchase. It is cheap insurance; this way, I don’t need to worry about it. Then the radiator neck cracked, so a new one was installed. Nothing much left to change out, but *not* due to random part swapping.

TXBill: Interesting. It appears that you have already been down this path. You have my empathy. Your experience with two cars also confirms my belief that this temperature is not normal. Again, some will argue that it is not in the red, therefore it is OK. That’s bull in my opinion. Something is wrong, and I want it fixed!

As far as your fan is concerned, this is interesting, as my 84 300 does not have a coolant temperature controlled fan. It is ONLY switched on via the thermo switch on the AC receiver-dryer. FYI, most (all?) of the coolant sensing thermo-switches that I have seen for fan control (on the 1970s through early 1980s MBZs) are calibrated to go on at 100 Centigrade (212 on your Fahrenheit gauge). There are dual stage ones, but that’s not applicable here. Let me know what year your 240 is, and I will look up the wiring diagram on my CD. Curiosity may have killed the cat, but maybe it can help cool your car down instead.

If your fan is not coming on with the AC, it will have an impact on AC cooling performance – there is not as much airflow over the condenser and therefore is not as much heat rejection as when the fan is running. This would also translate into a warmer running temp for the engine, as well. BUT it could be the chicken-egg syndrome here. IF there is not enough of a freon charge, the receiver may not get hot enough to trigger the switch (assuming that it is a heat triggered switch). If it is a pressure triggered switch, the same would apply. However, if there is *really* low pressure, the compressor will not engage at all due to the pressure switch which is there to protect the compressor. (And you thought that your post was long? My fingers are starting to bleed.


THANKS AGAIN EVERYONE!
RTH